The present invention relates to a device for receiving and treating blood, and more particularly, to a cardiotomy reservoir used in oxygenator circuits employed for extracorporeal circulation during open-heart surgery.
FIG. 1 shows a typical oxygenator circuit. Blood drained from the patient's vena cava passes through a venous drainage line 1, and blood from the surgical field passes from aspiration lines 2 and pumps 3 through a cardiotomy reservoir 4 and a line 5, before both entering a venous reservoir 7 held in a volume-regulating means 6. The blood is then sent by a pump 8 to an oxygenator 9 with a heat exchanger where it is oxygenated. The blood is then returned to the patient's body via an arterial infusion line 10.
Today, in addition to the defoaming and collection of blood aspirated from the surgical field, be this the thoracic cavity, cardiac cavity or another area, a great many cardiotomy reservoirs now in use have been provided also with a filter that removes foreign matter, such as tissue and microaggregates, which is sucked off together with blood from the operating field or is present in trasfusing blood. Such cardiotomy reservoirs thus have three important functions: filtration, defoaming, and blood collection. A number of problems have been identified with each of these functions in cardiotomy reservoirs in use today.
(1) Problems with filtration
When blood containing large amounts of foreign matter such as tissue and microaggregates is passed through the filter, premature clogging of the pores often arises before full use can be made of the filter. In the case of pleated filters, the filter is arranged cicumferentially, leaving an inner cavity in which blood pools. This makes it difficult to correctly determine the amount of blood within the reservoir. The blood quantity indicated by the scale on the reservoir housing thus differs from the actual amount of blood in the reservoir, making it impossible to get a true reading when necessary. Another problem is that good filter performance conflicts with the rapid separation of microbubbles from blood within the filter; getting a good balance of both properties is quite difficult.
(2) Problems with defoaming
Blood entering the cardiotomy reservoir is filtered and defoamed by a filter and defoaming agent before it is collected in the housing. However, in constructions where blood or priming fluid draining from the filter/defoamer drips directly onto the surface of defoamed blood pooled in the reservoir, bubbles can form again. Returning of blood containing microbubbles to the patient may possibly induce cerebropathy due to capillary occlusion or other complications.
To avoid such problems, some cardiotomy reservoirs are constructed with the filter/defoamer in contact with the base of the housing. However, this type of reservoir tends to give rise to "air blockage", meaning the trapping of air bubbles within the blood outlet port at the base of the housing and the tubing connected to this port. In an oxygenator circuit utilizing a membrane-type oxygenator and a closed venous reservoir, this presents a large danger of air entry into the venous reservoir, oxygenator, and patient. "Air blockage" here refers to the trapping of air within the tubing connected to the blood outlet port during a large influx of blood into the reservoir while the tubing is clamped off at the blood level in the tubing.
As for cardiotomy reservoirs having a filter built therein, the greater resistance of high-performance filters in use recently has made functionally imperative a construcion that defoams following filtration. Forced filtration under positive pressure occurs in filtering the incoming air-containing blood. However, with positive filtration, as the blood flow rate rises, the capacity of the defoamer to treat blood is at some point exceeded, and the air under pressure causes blood to spurt from the filter. Bubbles reform in the blood, which hampers subsequent defoaming operations. This is accompanied also by a large pressure rise in the filter, which tends to diminish filter performance. Attempts have been made to get around this problem by employing a cyclone-type air separation structure at the blood inlet port. However, such a setup is unable to cope with the influx of large volumes of air and invites a pressure rise, resulting in very poor defoaming performance.
(3) Problems with blood collection
The shape and structure of the housing also poses problems in terms of blood collection within the reservoir. Mold fabrication for a housing made of two flanged halves bonded at the center is easy. However, with this type of construction, in addition to the possibility of blood leaking from the junction, the calibration of volumetric graduations on labels is difficult, which tends to invite inaccurate calibration of the collected blood volume near the junction. Moreover, in some particular open-heart cases, blood volume measurements are ofter taken via a vent line to check for circulatory abnormalities and valvular incompetence. The heart is in an ischemic state. When pulmonary vein circulation is abnormal, blood pools in the left ventricle. The degree of abnormality can be determined by the amount of this pooled blood. The existence of valvular incompetence in the vena cava can also be determined by the amount of pooled blood in the left ventricle. A scale with fine graduations is required for the measurement of these volumes through a vent line. For this reason, users desire the base of the housing and the blood outlet port to be shaped such as to allow easy blood volume read-out.
Another consideration is blood and bubble separation. Among filters having the same surface area, a longer filter provides better separation. Hence, long filters reaching almost to the base of the housing are often used. However, in cardiotomy reservoirs of this type, the blood remains in extended contact with the filter material, resulting in significant complement activation and silicone extraction, both problems being of increased concern lately. Typical examples of cardiotomy reservoirs in which the filter extends down to the vicinity of the housing base are disclosed in U.S. Pat. Nos. 4,164,468 and 4,209,193.